Why zinc sulphate turns yellow?

In our sample cupboard, there are different batchs of zinc sulfate samples. The day before yesterday, I sorted all inventory samples out and found that some bags of zinc sulfate sample were stocked too long that appeared light yellow and their liquidity is not very good.

Generally speaking, as long as well preserved, zinc sulfate wouldn’t go bad. The mainly reason for zinc sulfate becoming yellow is that the package is not tight enough that make product long time exposed to the air. The products will combine with water in the air as hydrate. This affects fluency. Moreover, the impurity in the air would pollute it too. So the color of zinc sulfate slowly changes from white to light yellow.

On the other hand, the period of validity of zinc sulfate is about one year.  It s a normal phenomenon, when zinc sulfate samples becoming yellowing after it stocked over one year.

Another reason of making zinc sulfate yellowing is the content of Fe over common level. Therefore, in order to guarantee the quality of the zinc sulfate, such as the purity, liquidity and so on, seal the products well and stock in ventilated, cool warehouse. Remember to keep it away from fire and heat source and avoid to be mixed with other chemicals.